The present invention relates to a pedicle screw assembly.
A standard pedicle screw assembly comprises a screw having an externally threaded stem having in turn a head provided with parts allowing it to be secured to one end of a distraction rod. Two such screws inserted into respective vertebrae are secured to such a rod to distract and/or stabilize a spinal column after, for instance, a disk operation.
In a standard such assembly as described in German patent document 4,107,480 the head of the screw has a pair of outwardly projecting parallel ridges with overhanging inner edges. A cap formed with a pair of complementary inwardly open slots that can fit with these ridges has a part-cylindrical recess forming with another part-cylindrical recess on the head of the screw a seat for the rod. The screw is threaded into the vertebrae, an end of the rod is fitted to its outer end, the cap is then slid transverse to the screw axis and parallel to the rod over the rod to capture it, and finally a screw threaded into the cap is tightened to press the rod down against the head of the screw and thereby fix the rod, cap, and screw together.
Since the cap of this system must be slid transversely onto the head of the screw, it is necessary to open the operating field sufficiently to allow such movement. This not only increases the extent of the surgery, but makes it impossible to install two such screws close to each other, for instance on adjacent vertebrae. Furthermore in most systems the actual position of the screw is critical as the rod can only be fitted to it in a limited range of positions.
It is therefore an object of the present invention to provide an improved pedicle screw.
Another object is the provision of such an improved pedicle screw which overcomes the above-given disadvantages, that is which can be installed in very close quarters.
A pedicle-screw assembly has according to the invention a screw extending along an axis and having a threaded shaft and a round head, an inner cap part carried on the head and swivelable on the head, and an outer cap part forming with the inner cap part a seat adapted to hold an end of a distraction/stabilizing rod. The outer cap part has formations engageable in a snap fit over the inner cap part, and an element on the outer cap part can be tightened to fix the rod in the seat.
With this system once the pedicle screws are set, the rod ends can be fitted to the inner cap parts which can be swiveled to accommodate any necessary angular offset. Then the outer cap parts are snapped in place and the assembly is tightened to lock the rod in place. The outer cap part does not have to be slid transversely into position but is moved normally generally axially as it is snapped over the inner cap part, so that little clearance is needed and the operating field does not have to be expanded to accommodate such movement.
The screw head according to the invention has an inner region with a part-spherical outer surface and an end face lying on a plane substantially perpendicular to the screw axis. The head is formed on the end face with a central part-spherical bump centered on the axis. In addition the outer cap part is generally U-shaped and formed with a base forming the seat and carrying the fixing means and a pair of legs projecting parallel to the axis, flanking the cap inner part, and having ends engaged around sides of the cap inner part. Each of the legs has an outer end that hooks under an edge of the respective side of the cap inner part. The outer cap part is formed parallel to and adjacent each leg with a groove so that lateral elastic deformation of the leg is increased.
Interengaging formations on the cap parts block movement of the cap parts relative to each other transverse to the axis. Furthermore, the outer and inner cap surfaces have confronting side faces. The interengaging formations include at least one axially extending ridge on one of the faces and a complementary axially extending slot on the face confronting the one face. The one face having the ridge is on the inner cap part although of course the reverse system would be usable.
The head and inner cap part have surfaces that engage each other and that are roughened. The fixing means is a screw threaded into the outer cap part and tightenable to pull the roughened surfaces of the inner cap part and head axially together. Thus when the assembly is tightened, the cap can no longer swivel on the screw.
The outer and inner cap parts are formed with confronting part-cylindrical concavities forming the seat. The inner cap-part concave seat half has a throughgoing aperture. The fixing screw presses the rod in the seat against the bump. To maximize the range of relative angular position the screw is formed at a base of the head with an annular inset so as to allow a wide range of swiveling of the cap on the head.